Puzzle of hexagonal diamond at meteorite sites solved with help from University of Warwick physicists

Theoretical physicists at the University of Warwick have helped colleagues at Lawrence Livermore and Berkeley solve a puzzle dating from 1967 when a hexagonal form of diamond, later named lonsdaleite, was identified for the first time inside fragments of the Canyon Diablo meteorite, the asteroid that formed the Barringer Crater in Arizona in a violent impact.

It has been hypothesized that lonsdaleite forms when graphite-bearing meteors strike the Earth. The violent impact generates incredible heat and pressure, transforming the graphite into diamond while retaining the graphite's original hexagonal structure. However, despite numerous theoretical and limited experimental studies, crucial questions on the trasition pathways have remained unresolved for such short-living high-pressure environments occurring during meteor impacts. Particularly, the structural state immediately after the shock transit, the timescales involved and the influence of crystalline orientation.

In a new paper entitled “Nanosecond formation of diamond and lonsdaleite by shock compression of graphite” published this week by Nature Communications, a team of researchers, including scientists from the University of Warwick and led by researchers from Berkeley and the Lawrence Livermore National Laboratory (LLNL), provide new insight into the process of the shock-induced transition from graphite to diamond and lonsdaleite uniquely resolving the dynamics of the phase change.

Dr Dirk Gericke, a physicist from the University of Warwick Centre for Fusion, Space and Astrophysics and one of the researchers on the paper said:

“Such phase changes are very challenging for theory. Thus, these interesting experimental data are very valuable for benchmarking future simulations. It is amazing that we can now follow the evolution of the phase transition with brilliant x-ray probes from the Linear Coherent Light Source (LCLS); seeing the dynamics of the process gives much more insights than only having the initial and final state. This is a true new treasure for science.”

The experiments yielded unprecedented X-ray diffraction data on short timescales directly revealing the structural changes during shock compression of graphite. Diamond formation is abserved starting at pressures above 0.5 Mbar (1 Mbar = 1 million atmospheres). Moreover, the team observed the formation of lonsdaleite above 1.7 Mbar, for the first time resolving the process that has been proposed to explain the main natural occurrence of this crystal structure being close to meteor impact sites.

"Due to difficulties in creating lonsdaleite under static conditions, the overall existence of this crystal structure in nature has been questioned recently," said lead author Dominik Kraus. Kraus conducted this research while working as a University of California, Berkeley, Physics Department postdoc sited within LLNL's NIF & Photon Science directorate. He has now a position as a Helmholtz Young Investigator at Helmholtz-Zentrum Dresden-Rossendorf in Germany.

"However, static experiments cannot mimic fast dynamics such as those in violent meteor impact events," he said. "Here we show that we can indeed create a lonsdaleite structure during dynamic high-pressure events. This is interesting for modeling dynamic phase transitions in general, but also shows that the lonsdaleite found in nature could indeed serve as a marker for violent meteor impacts."

The experiments were conducted at the Matter at Extreme Conditions (MEC) experimental area at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory at Stanford. Graphite samples were shock-compressed to pressures of up to 2 million atmospheres (2 Mbar) to trigger the structural transitions from graphite to diamond and lonsdaleite. The phase changes in the high-pressure samples were probed with ultrafast (femtosecond) X-ray pulses created by LCLS.

According to Kraus, this was the very first in situ structure measurement of the shock-induced graphite to diamond transition. Before these experiments, all conclusions regarding this structural transition where based from the material that was recovered after applying the shock drive or after explosions.

Notes for Editors

The research was led by scientists from the University of California, Berkeley, and conducted by researchers from the University of Warwick, SLAC, the, the Max Planck Institute, Technical University Darmstadt, Helmholtz-Zentrum Dresden-Rossendorf, the University of Oxford and GSI.

The U.S. Department of Energy's Office of Science and the German Federal Ministry for Education and Research funded the work.

For further information please contact:

Dr Dirk Gericke
Centre for Fusion, Space and Astrophysics
Department of Physics
University of WarwickD.Gericke@warwick.ac.uk
A mobile/cell phone number for Dirk is also available from Peter Dunn whose details now follow: